There's actually significant variation between the cultivars. Here's a comparison of macronutrients in each food based on a 100 gram sample.
Total food energy (ie. calories) ranges from 25 kcal (cauliflower, cabbage) to 50 kcal (kale).
Similar variation is observed in macronutrient ratios. Protein content is lowest in cabbage (12.6%) and highest in kale and broccoli (21%). Fat content as a percentage of total food energy varied widely from cabbage (3.3%) to kale (15.8%).
Total and net carbohydrate contents are similar between all 6 vegetables. Brussels sprouts provided the most fibre (3.8 grams) while cauliflower provided the least (2.0 grams).
A large difference in omega-3 fatty acids was observed, with kale providing the most (180 mg) while broccoli and cauliflower provided the least (20 mg).
Protein amino acid composition also varied considerably. For example, lysine content was high in cauliflower (220 mg) but low in cabbage (40 mg).
Because macronutrient contents varied considerably, I'll try to compare vitamins more fairly by considering a 100 kcal portion of each food.
Some B vitamins were present in similar contents, but kohlrabi was especially low in B2 (riboflavin) and a wide range was observed for B5 (pantothenic acid) with cauliflower providing the most (2.7 mg) and kale the least (0.2 mg).
Folate contents were similar, but kohlrabi (59 µg) was much lower than average (mean=179, median=179).
Vitamin A content (in the form of carotenoids, especially beta carotene) was extremely variable, with kale providing the most (20,000 IU) and kohlrabi the least (130 IU).
Vitamin E content was extremely variable with broccoli providing the most (4.2 mg) and cauliflower the least (0.3 mg).
Vitamin K content was extremely variable with kale providing the most (1438 µg) and kohlrabi the least (0.4 µg).
No single variety was best at providing all vitamins.
A few minerals (iron, magnesium, manganese, phosphorus, potassium, sodium) had low variation between plants, with only about a 2:1 difference from most to least. However, some minerals did vary considerably more.
Kale has an unusually high calcium content (306 mg) compared to the average (mean=146 mg, median=118 mg).
Kohlrabi and especially kale have high copper content (0.5 and 3.1 mg, respectively) while all the rest have 0.2 mg or less.
Selenium content was low in cabbage and kale but high in broccoli and brussels sprouts.
Zinc content was mostly consistent around 0.7-1.2 mg, except for kohlrabi which was especially low at 0.1 mg.
No, cruciferous vegetables do not have very similar nutritional profiles despite the fact that they are all varieties of the same original plant. It is best to consume cruciferous vegetables in combination in order to get a variety of vitamins and minerals.